It is common for many hydrocarbon reservoirs to produce gas which has low BTU content and high carbon dioxide concentrations (typically called "waste gas") along with the more valuable hydrocarbons. It is also common for such waste gas to be contaminated with trace amounts of hydrogen sulfide which, as is well known, is particularly hazardous to the environment. Accordingly, disposal of the waste gas after it is separated from the other produced fluids presents a real problem in commercially producing such reservoirs.
Many oilfield operators in California have difficulty in disposing of low-BTU produced gases. Regulations regarding waste gas emissions grow more strict each year. In order to avoid the environmental problems associated with venting these gases, several techniques have been proposed for disposing of waste gas in an ecologically-acceptable manner. One such well known disposal technique involves mixing the waste gas with a higher BTU gas and then burning the resulting mixture as fuel at or near the production site, e.g. fuel to generate steam for reinjection into the producing formation to heat heavy oil and the like. Burning of waste gas as fuel, however, normally requires that the gas first be treated to remove any hydrogen sulfide that may be present in the waste gas. This requires relatively expensive equipment which, in turn, routinely experiences substantial downtime (e.g. 10% or more) during normal operation. Since it is desirable to continue the production of hydrocarbons even during periods when the gas treating equipment is down, disposal of the waste gas again presents a problem since it can not be burned during these periods.
Other proposed techniques for disposing of waste gas have involved injecting the waste gas through an injector well directly into a depleted or spent zone of the formation from which the gas was produced. For example, in a heavy oil reservoir where steam is injected to heat the oil to reduce its viscosity, it has been proposed that waste gas from the production be injected into the steamed-out, already produced zones of the formation. However, due to the high diffusivity of the waste gas within a typical producing formation, the waste gas has a tendency to quickly and easily flow from the steamed-out zone, through the hydrocarbon producing zone, and to the production well(s) to be produced again with the hydrocarbons. This "early breakthrough" of the waste gas at the producing wells seriously detracts from the apparent benefits otherwise derived from this disposal technique since the waste gas is "reproduced" and accordingly, has to be "redisposed of" which, in turn, substantially adds both to the problems of handling the waste gas and to the costs involved.
Therefore, what is needed is a method for disposing of waste gas in a swept or depleted zone where the gas is reacted within the formation to remove harmful products therefrom so as to produce a waste gas to the surface that is substantially free of harmful products.